This invention relates to the oxidation of the spent caustic occurring in the process of desalting crude oil.
Among the most common spent caustics produced by industry are those containing sodium sulfide, sodium bisulfide, sodium bicarbonate, sodium naphthenates, and sodium cresylates. If the above spent caustics are discharged directly into plant effluent waters, they place a burdensome oxygen demand on the receiving waters. Fish and other aquatic populations are adversely affected, and water may become unsafe for human consumption or it may gain a distasteful flavor. For these and other reasons, spent caustics are normally converted to a form that minimizes these problems.
Systems for oxidizing these spent caustic have been around for many years, but such systems have required complex, expensive multi-vessel facilities. For example, the Shell design described in U.S. Pat. No. 2,869,844, "Treating Liquid With Gas", Issued Jan. 20, 1959, by S. B. Thomas, incorporated by reference herein, and also described in an article by J. D. Martin and L. D. Levanas, "Air Oxidation of Sulfide In Process Water", Division of Refining, Vol. 42, pp. 392-98 (1962), required a feed/product exchanger for a gas/liquid mixture and three processing vessels: an oxidation column, a vessel for gas/liquid separation, and an offgas furnace. A flow diagram of the Shell process is provided in the article as FIG. 1 on page 393. A similar system is described by Milton R. Beychok in Aqueous Wastes From Petroleum and Petrochemical Plants, (2nd ed. 1973 John Wiley & Sons 1973) in a chapter entitled "Treatment Methods," pp. 156-266. On page 202, in particular, FIG. 30 shows a diagram of a typical oxidizing unit which is substantially the same as the Shell design. The chapter of this book entitled "Treatment Methods" is incorporated herein as if fully set out verbatim. Merlehem Company also previously designed systems requiting a feed/product heat exchanger for a gas/liquid mixture and three vessels: an oxidation tower, an offgas K.O. drum for gas/liquid separation, and an offgas treater. This is described in F. J. Suarez, "Safe Waste Caustic Disposal", Hydrocarbon Technology International '89/'90, which is incorporated herein by reference. The Suarez article contains two flow diagrams as FIGS. 1 and 2 that show the previous equipment and process required for oxidation of spent caustic. FIG. 1 includes a neutralization system, as well, for adjusting the pH of oxidized caustic. The offgas treater is not shown in either figure. Other oxidation processes have been developed that are even more complex. For example, in U.S. Pat. No. 4,384,959, Baurer, et al, "Wet Oxidation Process Utilizing Dilution of Oxygen", issued May 24, 1983, a method for wet oxidation of aqueous liquor containing combustible matter is described which requires a separate vessel for gas-liquid separation; a pressure control valve to reduce the pressure of cooled oxidized liquor and offgases; a feed/product exchanger; and means for injection of inert gas into the waste liquor to dilute the oxygen in the gas phase. Another example appears in U.S. Pat. No. 4,812,243, Brandenburg, et al., "Process for Treating Caustic Metal Wastes", issued Mar. 14, 1989, incorporated herein by reference, which describes a method for treating caustic cyanide and metal waste by wet air oxidation.
Thus, a system for oxidizing caustic that is simpler to build and operate, and eliminates use of a costly feed/product mixed phase exchanger to cool the oxidized caustic and gas released from the oxidation tower is needed in the industry. Furthermore, an advancement over the present technology, where caustic and hot gas would be separated before being transported from the oxidizing tower would be advantageous because it would eliminate special metallurgical and surface area requirements for downstream equipment such as an exchanger. F. J. Suarez, "Safe Waste Caustic Disposal", Hydrocarbon Technology International '89/'90, p. 2 of the reprinted article.